Fatigue Life Prediction for a Material Under Programable Loading Using the Cyclic Stress--Strain Properties

A method of estimating the fatigue life of a material under repeated loading using a block of ordered harmonic cycles of various amplitudes is presented. From the accumulated hysteresis energy the hypothesis of fatigue damage accumulation is derived, taking into account the characteristics of the material under cyclic load. The method was verified using low-carbon steel specimens and the results are in good agreement with the experimental results. Various modifications are presented, utilizing either the parameters of the cyclic deformation (stress--strain) curve for materials which are cyclically stable and have a short fatigue-hardening (or fatigue-softening) stage or the parameters of the Manson--Coffin and Wohler curves. The effects that the amplitude (for amplitudes below the fatigue limit), the arrangement of the cycles in the block and the other load characteristics have on the calculation are all analyzed. The theoretical and experimental results are compared with the Palmgren--Miner hypothesis and it is demonstrated that this hypothesis is only a special case of the hypothesis presented. The method can also be applied to a random operational loading process, provided that it has been transformed into a simple spectrum of harmonic cycles, i.e. a programed loading block (this is best achieved with the "rain flow" method). 7 ref.--AA